socket wrench

Short definition from Wiktionary

Noun

1. 1
   A type of wrench (spanner) featuring multiple removable and interchangeable socket heads (sockets), each designed to fit a particular size of bolt head or other fastener, which can be attached by the drive to a ratcheting handle, breaker bar, or other driver.
2. 2
   Any wrench, including a nonratcheting one with an integral (nonremovable) head, that has an internal/concave feature (a socket) that engages the head of an external-wrenching fastener.